(1) Field of the Invention
This invention relates generally to digital cameras and relates more specifically to alignment of camera modules and measurement of sharpness distribution.
(2) Description of the Prior Art
Alignment of camera modules is often done using iterative approaches where alignment steps and measurements of the sharpness distribution are performed and iteratively refined until an optimum is found. Measurement of the sharpness distribution can be done either by visual inspection by a human operator or by time consuming objective measurements like Module Transfer Function (MTF) measurements. MTF is the spatial frequency response of an imaging system or a component; it is the contrast at a given spatial frequency relative to low frequencies. This is a time consuming process that should ideally be shortened for production systems.
It is a challenge to accomplish an optical adjustment of cameras with automatic equipment combined with a real time display of values during adjustment.
Solutions dealing with adjustment of cameras are described in the following patents:
U.S. Patent (U.S. Pat. No. 6,628,339 to Ferland et al.) teaches a digital camera having an image sensor and a lens defining an optical axis for focusing an image upon a focal plane, an image sensor mount includes the digital camera having a housing for mounting the lens and defining a first opening through which an image can be focused by the lens and transmitted along the optical axis, and an image sensor assembly including the image sensor attached to an image sensor mounting plate. The image sensor mount further includes a camera mounting plate having first and second surfaces, the first surface being fixedly attached to the camera housing and defining a second opening through the first and second surfaces which is aligned with the first opening, the camera mounting plate including at least three spaced-apart alignment elements on the second surface, the alignment elements being located at a predetermined position relative to the lens; and an image sensor mounting plate for attaching the image sensor assembly to the camera mounting plate so that the image sensor mounting plate engages the alignment elements to center the image sensor with respect to the optical axis at the focal plane.
U.S. Patent Publication (US 2008/0246873 to Kinoshita) discloses a camera module including a lens unit which includes a lens and a lens holder holding the lens therein, and an image sensing unit which includes a solid-state image sensor and a transparent lid section which is arranged so as to face the receiving surface of the solid-state image sensor, provided with a space S therebetween. The transparent lid section and the lens are independent of each other, and alignment of the lens with respect to the solid-state image sensor is performed by mounting the lens on the transparent lid section. Thus, it is possible to provide a solid-state image-sensing device having high lens alignment precision with respect to a solid-state image sensor.
U.S. Patent Publication (US 2006/0108518 to Lee et al.) describes a structure for calibrating the alignment between a lens set and an image sensor in the process of packaging such as an electric micro-optic module (EMOM) or compact camera module (CCM) by using a matching structure like sawteeth or V-grooves at contact surfaces or edges. Random variations due to manufacturing process can be averaged out by a plurality of the V-grooves and high precision can be obtained by the present invention.